STEVE HORRELL / SCIENCE PHOTO LIBRARY STEVE HORRELL / SCIENCE PHOTO LIBRARY
Halogen lamp. Close-up of a halogen lamp. Halogen light bulbs run at higher temperatures than normal domestic bulbs, enabling them to produce a more natural colour balance. To prevent fast depletion of the tungsten filament, the bulb has an inner quartz glass envelope filled with low-pressure iodine gas. The iodine reacts with tungsten atoms sublimating from the filament, preventing them from depositing on the glass. When the bulb is switched off, the tungsten is redeposited onto the filament. Small metal coils inside the inner envelope set up an electrical field which repels tungsten ions, further reducing loss of the metal from the filament.
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